A number of U.S. patents have been issued for circular aircraft configurations, embodying a variety of approaches to the generation of lift and control of attitude. Some of these patents teach levitation of a body by the direct thrust of single or multiple jets: more recent patents (e.g. U.S. Pat. Nos. 2,978,206, 2,996,266, 3,041,009, 3,276,723, 3,405,889, 3,592,413, 3,612,445, 3,697,020, and 3,785,592) utilize the flow of jets over the surface of wing-like structures. A review of these patents reveals that none of them contains predictions of load capacity, dimensions, power requirements, or the lift which would be generated by the configurations proposed. A review of the technical literature indicates that there have been no actual experiments with such craft, either tethered or in free flight.
During recent developments of VTOL and STOL aircraft, a great deal of analytical and experimental work has been performed on the behavior of jets exhausting parallel to lifting surfaces. For example, in a five-year study done by Rockwell ("A Study of Wall Jets and Tangentially Blown Wings", N. D. Malmuth, W. D. Murphy and J. D. Cole, Rockwell International Science Center--Report of ONR Contract N-0014-76-C-0350) the analysis of a jet exhausting over the upper surface of a wing was advanced to the point where subsonic and supersonic flows over a lifting surface could be modelled in detail, but no experimental verification was carried out. In their conclusions, the writers projected that more accurate results awaited further development of parabolic marching techniques.
At the David Taylor Naval Ship R&D Center, the controlled deflection of a large jet by a thin annular jet was investigated experimentally, ("Investigation of Parameters Influencing the Deflection of a Thick Wall Jet by a Thin Wall Jet Coflowing over a Rounded Corner", Gregory G. Huson, David Taylor Naval Ship R&D Center--Report DTNSRDC/ASED-83/10) with the indication that this is an effective means of vectoring thrust of a jet exhausting over the surface of a lifting body. Among the conclusions reached were that the effectiveness of this technique for thrust vectoring would be sensitive to the radius of curvature of the lifting surface, and to the relative upstream location of the main and control jets.
Further advances in the theory of turbulent wall jets were made in a study funded by ONR, conducted by Grumman Aircraft Corporation ("Theoretical Aerodynamics of Jets in Ground Effect Phase V--Asymptotic Theory of Turbulent Wall Jets", R. E. Melnik and A. Rubel, Grumman Aerospace Corporation--Final Report, Contract N00014-81-C-0549). The authors recommended experimental verification, but concluded that flows adjacent to a curved surface were difficult, if not impossible, to model. Papers in two recent AGARD Conferences have dealt with the subject of lift production by the combination of jets and adjacent surfaces, with particular application to VTOL and STOL aircraft. A session of the November 1981 conference was devoted to the topic "Jet Interactions with Neighboring Surfaces". In this session investigators from the University of Virginia ("An Experimental Investigation of an Upper Surface Blowing Configuration", G. D. Catalano, J. B. Morton and R. R. Humphris--AGARD November 1981) reported on laser velocimetry experiments performed with jets adjacent to a flat plate and to a flap upper surface. They were unable to project whether the jet would attach to the surface of the flap under static conditions.
The May 1984 AGARD Conference dealt with enhancement of lift by various means. One of the pertinent papers ("Modelling Circulation Control by Blowing", M. M. Soliman, R. V. Smith and I. C. Cheeseman, AGARD May 1984) was a study by Westland Helicopters of the lift and drag reduction effects of circulation control by blowing, on flows around circular bodies. The authors projected that the theory they developed would also predict the effects on lift of circulation control by blowing, for airfoils of any shape.
Applicants have been unable to find any references which deal with analytical prediction of lift for a circular aircraft, and no reports of experimental measurements on such craft. We decided to study the lifting characteristics of such a craft experimentally, and have developed a novel configuration which has a useful payload, can be controlled with stability and has operational utility. The report of our experiments is contained in "Circular Airplane Investigation", Final Report on Contract F33657-87-C-2164, Vatell Corporation, Apr. 18, 1988.
The principal difference between a helicopter and a circular airplane is in mechanical complexity. The rotor or rotors of a helicopter turn at a speed which is slow compared to that of the engine, and a gearbox is required to multiply the torque and reduce the speed of the engine. To control flight the pitch of the helicopter blades must be varied in two modes; all at once, to establish overall lift, and cyclically, to produce a lift vector and compensate for the effects on lift of horizontal motion through the air. The rotating main rotor blades produce a large torque in the horizontal plane on the helicopter, and in a single rotor craft this must be opposed by a separately controlled tail rotor. Multiple rotor helicopters balance the torque of one main rotor against that of the other to achieve cancellation and control vehicle rotation. The reference "New Aerodynamic Design of the Fenestron for Improved Performance", A. Vuillet and F. Morelli, AGARD Conference Proceedings No. 423, October, 1986 contains the statement: "The number of helicopters crashed due to failed or impacted tail rotors is about 0.15 per 10,000 hours of flight in the accident log book, as compared to a registered overall number of accident of 0.71 per 10,000 hrs of flight"
In "Summary of Drive-Train Component Technology in Helicopters", Gilbert J. Weden and John J. Coy, AGARD Conference Proceedings No. 369, January 1985, problems with the power transmission systems of helicopters are summarized: "Achievement of long-lived, reliable power transfer systems can be difficult to achieve and today's helicopters are one of the most severe applications of this technology. Helicopters (sometimes referred to as flying fatigue machines) present the ultimate test of materials and designs for reliability. The many failure mechanisms for bearing and gears must be weighed against anticipated loads which are not know with certainty. In addition to known classical modes of failure, such as pitting, scoring, and bending fatigue, there are unanticipated events that can ground helicopters. Things like sudden leaks producing low oil levels, undetected contamination of lubricant, and poor maintenance practices can severely lower the reliability of the mechanical components of the transmission."
By contrast, a circular airplane of the type we propose can be designed to operate with direct coupling (no gears) between its engine and fan, and has no rotor and no pitch controls. Any torque produced by the air flow which produces lift may be minimized by redirecting it with airfoils or vanes, so there is no need for a tail rotor or second main rotor. Flight control surfaces can be simple gates or dampers which modify the velocity distribution external to the craft. Rotation of the craft can be controlled by simple vanes which divert the main flow horizontally. Lift is controlled by engine speed, or for more rapid response may also be controlled by throttling the main lifting jet flow.
Applicants' circular airplane should be able to achieve a level of reliability which is close to that of its engine alone, because the components added for flight control are not highly stressed, and may even be designed for aerodynamic redundancy. In contrast, the flight control elements of the helicopter are among its most highly stressed, and have consequently high failure rates. An internal combustion engine power plant for a circular airplane would have the advantage that it operates at speeds which will allow direct coupling to the fan, although turboshaft or turbofan engines may ultimately prove to be practical, especially for larger craft.
A helicopter has its center of lift well above the center of gravity of the craft, and this produces a large righting moment which must be overcome by the cyclic pitch controls for any change in attitude. In applicants' circular airplane, the center of lift will be near the center of gravity, and the righting moment which must be overcome by vectoring of lift will be quite small.
The moment of inertia of the helicopter main rotor is quite large, and gyroscopic effects have a pronounced effect on maneuvering. In contrast, the moment of inertia of applicants' circular airplane will be much smaller, and only the rotating parts of the engine produce gyroscopic effects.
Because the helicopter blade moves a large volume of air at low pressure, it is efficient in generating lift. The circular airplane will be less efficient, because it moves a smaller volume of air at a higher velocity and pressure. The payload of a circular airplane will be less than that of a helicopter with the same fuel rate.
In general a helicopter is much more observable than a circular airplane will be, because of its greater size and the large rotating blade assembly. The circular airplane should have a small infrared signature because its heated exhaust can be mixed with a much larger volume of air. The vehicle body may be constructed of reinforced plastics which have a low radar reflectivity. The noise of a circular airplane will be limited to that of its engine and the fan it drives, and vibration can be minimal, depending on how well these components are balanced. Noise reaching the ground should be extremely low, in fact this vehicle may be almost as quiet as a glider because the body will shield engine noise from the ground and the jet velocity around the body of the vehicle will be relatively low. All high speed, turbulent mixing will occur above the vehicle.
In "Minimisation of Helicopter Vibration Through Active Control of Structural Response", S. P. King, A. E. Staple, AGARD Conference Proceedings No. 423, October, 1986 the problem with vibration in helicopters is succinctly described, "The control of vibration has been and remains, a problem for all rotary winged vehicles. Considerable efforts have been expended over many years in attempts to reduce vibration to acceptable levels. On the helicopter there are many sources of vibration, but the most important component is generated by the main rotor and occurs at a frequency (bR) equal to the product of the number of blades (b) and the rotor speed (R). This blade passing frequency vibration is an inherent consequence of driving a rotor edgewise through the air, and can never be completely eliminated, although the magnitude of the rotor excitation can be controlled by careful rotor system design. The response of the air frame is also sensitive to the dynamic characteristics of the fuselage, and again careful design can minimize the response. As understanding of the nature of the problem has increased, and the ability to predict the dynamic response of both rotor and airframe has improved, it has become possible to design a helicopter for low vibration, or at the very least to avoid those problems which have led to very high vibration in the past. The trend for increased cruise speed, and mission endurance has, however, aggravated the problem, since the magnitude of the rotor vibratory loads increases with speed, and the effect of vibration on human fatigue is proportional to exposure time."
Little can be said about the relative speeds in horizontal flight of helicopters and circular airplanes. Helicopters have a fundamental limitation: the backwardly moving rotor blades produce less lift than those moving in the direction of travel. At some limiting speed the backwardly moving blades will stall, and the helicopter cannot approach this speed with safety.
While circular airplanes may not have advantages over helicopters in vertical takeoff and landing, they may be able to make the transition to horizontal flight more easily, and may be ultimately capable of higher speeds than helicopters. Horizontal flight characteristics of applicants' circular airplane are unknown, but its speed will probably be limited to less than the exit velocity of the main jet. With certain vehicle profiles a scheme for diverting all of the flow to one side of the vehicle may make it possible to achieve high speed horizontal flight, but this remains to be explored.
The helicopter applications for which applicants' circular airplane may be most attractive are those which require:
(1) reliability; PA1 (2) maneuverability; PA1 (3) small payload; PA1 (4) low vibration; and PA1 (5) low observability.
Among military applications the one which immediately stands out is the battlefield reconnaissance mission. Here the ability to operate from a small base is crucial, and the circular airplane has a real advantage. It will be able to take off and land in a space not much larger than its own area with greater safety than a helicopter, whose rotating blades are extremely hazardous to personnel.
Reconnaissance missions may be separated into manned and unmanned types. In the former, the payload consists of a pilot, sensors, computers and communications equipment. A typical payload might be 500 to 1000 pounds, and flight times of 1-3 hours are typical of tactical applications, mostly in slow speed, level flight. There is a strong trend towards the use of remotely piloted vehicles for reconnaissance. In these applications the payloads are smaller, but other requirements are the same. "Mini-helicopters", powered by internal combustion engines, have been developed for this use, but they have all the complexity, and most of the control problems, of larger helicopters and are extremely limited in payload and endurance.
In the commercial and industrial marketplace the prospects for circular airplane applications are similar. While a circular airplane may never be used for heavy lifting, there are surveillance and monitoring tasks now performed by helicopters which it could do better. Providing a truly maneuverable but steady platform for aerial photography, a remotely piloted circular airplane could be launched from the back of a pickup truck and directed to take photographs from a variety of angles. It could be used for observation of forest fires, natural and man-induced disasters, and routine traffic and crime surveillance. With a laser beam projected from a ground station, this type of craft could be directed to a fixed position over its objective, perform its mission, then be brought back to the launching site in a "beam riding" mode with the exposed film or recorded data.
These and other advantages are achieved in applicants' circular airplane configuration, which is herein disclosed and described in detail.